Some of the most recognizable artwork depicting the American West is heading to auction at Christie’s, where dozens of pieces from billionaire Bill Koch’s collection are expected to fetch at least $50 million. The in-person “Visions of the West” sale will take place in New York over two sessions beginning Jan. 20, with the final lots offered — appropriately — at high noon the following day. Koch’s holdings include major works by Frederic Remington, Charles Marion Russell and Albert Bierstadt, artists whose images of cowboys, Native Americans and sweeping landscapes helped define how generations came to picture the American frontier.